OF MICROSCOPIC OBJECTS. 61 



cover, and carefully place upon it a small streak of Canada balsam 

 from one end to the other. This, if laid on the cell with one 

 edge first, and then gradually lowered until it lies flat, will drive 

 all the air before it, and prevent any bubbles from being included 

 in the cell. It can be easily put on so neatly as to require no 

 cleaning when dry. If the cover is pressed down too rapidly, the 

 balsam will flow over it, and require to be cleaned off' when hard- 

 ened, for it cannot be done safely while fluid at the edges." 



Sometimes with every care bubbles are enclosed in the balsam 

 injuring objects which are perhaps rare and valuable. The whole 

 slide must then be immersed in turpentine until the cover is 

 removed by the solution of the balsam ; and the object must be 

 cleansed by a similar steeping. It may then be remounted as if 

 new in the manner before described. 



The balsam and chloroform described in Chapter I. is thus used ; 

 and where the object is thin, the mounting is very easily accom- 

 plished. When the object is laid upon the slide with a piece of 

 glass upon it, and the balsam and chloroform placed at the edge 

 of the cover, the mixture will gradually flow into the space be- 

 twixt the glasses until the object is surrounded by it, and the un- 

 occupied portion filled. The chloroform will evaporate so quickly 

 that the outer edge will become hard in a very short time, when 

 it may be cleaned in the ordinary way. Sometimes the balsam is 

 dissolved in the chloroform without being first hardened; but 

 this is only to render it more fluid, and so give the operator less 

 chance of leaving bubbles in the finished slide, as the thicker the 

 medium is, the more difficult is it to get rid of these intruders. 



It has been before mentioned that some have objected to chlo- 

 roform and balsam, believing that it became clouded after a cer- 

 tain time. Perhaps this may be accounted for in part by the fact 

 that almost all objects have a certain amount of dampness in them. 

 Others are kept in some preservative liquid until the time of 

 mounting, and these liquids generally contain certain salts 

 (Chapter IV.). If this dampness, as well as all traces of these 

 salts, however small, are not totally removed the former by dry- 

 ing, the latter by repeated washings the addition of chloroform 

 will render the balsam much more liable to the cloudiness than 

 when balsam alone was used, as before mentioned. 



